The invention relates to a process for the production of a stove-enameled moulded component from a double-layer plate having an intermediate layer of an epoxy resin in the form of a hot melt glued to its cover plates.
Such moulded components used, for example, in motor vehicle body construction, must have as low a weight per unit area as possible, accompanied by high buckling resistance. The double-layer plate must also be satisfactorily shapeable, more particularly be satisfactorily deep-drawable. Lastly, the moulded component must cause no problems during enameling, more particularly cathodic immersion enameling or in the subsequent stoving of the enamel.
The problem is known from U.S. Pat. No. 4,594,292 A1 that in a double-layer plate with a resin intermediate layer, glued to the cover plates, the adhesive connection between the intermediate layer and the cover plates is cancelled out by loosening with heavy deformations. To prevent this undesirable event while at the same time ensuring that during shaping the intermediate layer does not press so heavily against the cover plates that their outer zones become cracked, the intermediate layer consists of three different resin layers each having different properties. The core layer behaves plastically, while the outer layers consist of a ductile resin which does not flow under the shaping loadings occurring and therefore does not press against the cover plates of the double-layer plate. In this way extreme deformations with bending by up to 180xc2x0 are supposed to be achieved without the intermediate layer becoming loosened from the cover plate and/or cracking taking place in the cover plates.
However, moulded components produced from such a double-layer plates are unsuitable for stove enameling, since the outer layers of the intermediate layer consist of a mixture of polyethylene and polypropylenexe2x80x94i.e., thermoplastics which become liquid at temperatures above 160xc2x0 C. During the stoving of the enamel during stoving enameling, which is typically carried out at temperatures of 180xc2x0 C. and above for a minimum duration of 30 minutes, the intermediate layer would partially run out, and the adhesive connection between the cover plates and the intermediate layer, important for the buckling resistance of the moulded component, would be lost.
A double-layer plate is also known (EP 0 598 428 A1) in which an intermediate layer of polypropylene is glued by adhesive to the cover plates. Such a double-layer is supposed to be suitable for cold shaping to give a moulded component, which is supposed to have high dimensional stability after heat treatment at least 135xc2x0 C. Since polypropylene is a thermoplastic which becomes liquid at temperatures above 160xc2x0 C., the same thing applies to such a component as to the previously-mentioned one, namely that the adhesive connection between the cover plates and the intermediate layer is lost with the treatment time of 30 minutes at a minimum temperature of 180xc2x0 C., as is customary in enamel stoving.
It is an object of the invention to provide a process for the production of a stove-enameled moulded component from a double-layer plate having an intermediate layer of epoxy resin in the form of a hot melt which is glued to its cover plates.
It is also an object of the invention to provide a double-layer plate which is both deep-drawable and also suitable for stove-enameling. More particularly the double-layer plate is suitable for a production process according to the invention of stove-enameled moulded component.
This problem is solved by the following process steps:
a) after the introduction of the intermediate layer of an epoxy resin, which polymerises (cross-links) only at enamel stoving temperatures above 180xc2x0 C., between the cover plates and the spatial fixing of the cover plates in relation to one another, the double-layer plate is transformed into the shape of the component with the epoxy resin coherent but not yet polymerised (cross-linked),
b) after the shaped component has been enameled, the enamel is stoved at a temperature at which also the polymerisation (cross-linkage) of the epoxy resin takes place simultaneously.
In the process according to the invention, the selection of a particular epoxy resin in a particular state ensures that during the shaping of the double-layer plate to give the moulded component, more particularly by deep-drawing, since the epoxy resin is not yet cross-linked the intermediate layer shares in the deformations, and the cover plates are not impermissibly heavily loaded and/or the cover plates do not become loosened from the intermediate layer. This at the same time ensures that due to its solid state during shaping, the material of the intermediate layer is not forced out between the cover plates. It also makes sure that the epoxy resin does not melt and run out at the conventional higher, more prolonged temperatures of stoving, but polymerises, so that the final firm adhesion of the cover plates to the intermediate layer only then takes place, and after the cooling of the epoxy resin the shaped component acquires optimum buckling resistance.
A double-layer plate having an intermediate layer of epoxy resin and the following recipe was found to be particularly suitable:
Composition of the bonding agent system (in % by weight):
The additives enable adjustment to be made to the Theological properties of the material of the intermediate layer. For example, this can be achieved by means of a material, such as highly disperse silica, which reduces flow behaviour at elevated temperatures. However, control can also be effectively obtained by the use of micro hollow balls. With these additives, the material of the intermediate layer can be thixotropically adjusted in the temperature range up to 230xc2x0 C. This ensures that during enamel staving the material of the intermediate layer does not run out between the cover plates.
There is no need for an accelerator, since the material hardens only during the enamel staving. A suitable hardener is, for example, dicyandiamide. Although it is basically possible to use exclusively epoxy resin as the intermediate layer, this would involve very high material costs and a heavy weight of the component. To reduce weight, according to one feature of the invention fillers having a density which is substantially lower than the density of the epoxy resin are admixed to the epoxy resin mixture. Various materials are suitable for this purpose, such as cellulose fibres, wood dust, porous recycling glass, but more particularly micro hollow balls, preferably having a diameter of 10 to 100 xcexcm. Suitable materials which can be used for the micro hollow balls are thermoplastics, duroplastics, ceramic materials or glass. The best results are obtained using glass hollow balls. In this way densities of 0.6 g/cm3 can be obtained in the intermediate layer.
The buckling resistance of the moulded component depends also inter alia on the adhesion of the intermediate layer to the cover plates. This applies both to moulded components made from double-layer plates with flat cover surfaces, and also moulded components consisting of cover plates with a knobbed plate. In the latter case the number of knobs can be reduced in proportion as the adhesion of the intermediate layer to the cover plates increases. To improve adhesion, and adhesion-enhancing material, such as talc, can be admixed to the epoxy resin mixture. Buckling resistance can be further improved by the admixture to the epoxy resin mixture of a cohesive adhesion enhancing material, such as wollastonite. Lastly, at least one corrosion inhibitor is admixed to the epoxy resin mixture to prevent the occurrence of corrosion on the inner side of the inaccessible cover plates.
The intermediate layer can be produced in various ways.
In a first alternative the intermediate layer used is a foil to which the cover plates are glued. Gluing can be performed by means of a primer which is, for example, painted on to the cover plates and which glues the foil to the cover plates under pressure more particularly after the cover plates have been applied. The use of a glued-on foil is above all advantageous if the cover plates used are flat plates. In that case, the glued-on foil also serves for the spatial fixing of the cover plates during further processing.
However, also alternatively the epoxy resin mixture can be extruded as the intermediate layer directly onto a cover plate.
According to another possible feature of the invention the cover plates are glued to the intermediate layer by the heating of the composite formed by the cover plates and the intermediate layer to an epoxy resin melting temperature (approximately 100xc2x0 C.) lower than the cross-linkage temperature, for example, in a hot press. At the same time, however, as yet no polymerisation (cross-linkage) of the epoxy resin takes place.
If the cover plates used are flat plates, the cover plates are spatially fixed by the intermediate layer glued to the cover plates. However, if a knobbed plate and a flat plate are used, the knobbed plate is fixed to the flat plate by its knobs being welded on with the intermediate layer applied. Preferably welding-on is performed by resistance welding. The intermediate layer, laid or extruded on as a closed foil, is removed in the zone of the knobs, being displaced by electrode pressure. This means that during welding-on, the epoxy resin yields to the pressure. During this, otherwise than with an intermediate layer perforated for the knobs to pass through, advantageously the gap between the cover plates remains filled to its full volume. The removal of epoxy resin from the zone of the knobs is facilitated if pressure and heat are applied simultaneously. Heating can be provided, for example, by a heatable welding table top, via which a flat plate lying thereon and therefore also the intermediate layer are heated.